Lguest: The Simple x86 Hypervisor

Lguest allows you to run multiple copies of the same 32-bit kernel:
simply modprobe lg, then run Documentation/lguest/lguest to create a new
guest.

I suggest you try this yourself - lguest is incredibly easy to get up
and running. It's also quite useful: I can test-boot kernels with it
in less than a second, or about 10x faster than basic qemu, and 100x
faster than a real boot. And as it uses a pty as console, you can do
things like pipe it through grep.